The invention relates to apparatus for measuring the pH of a liquid. More specifically, the invention relates to apparatus capable of measuring the pH of a liquid at high temperatures and pressures.
At temperatures of not more than about 100.degree. C. and at pressure not far removed from atmospheric pressure, the pH of a liquid may be readily determined by means of a conventional glass electrode. However, at temperatures substantially above 100.degree. C., glass electrodes are unusable because the delicate glass membrane upon which the electrode depends is subject to severe chemical corrosion.
It has recently been discovered that certain ceramic membranes may be used in place of glass electrodes for pH measurement in high temperature liquids. A potential difference exists across these ceramic membranes that depends upon the pH of the medium in a manner that is analogous to that for the glass membrane of a glass electrode. Moreover, such ceramic membranes are chemically stable to temperatures in excess of 340.degree. C. and are thus usable for the measurement of pH at such elevated temperatures. A pH electrode using such a ceramic membrane is described in U.S. Pat. No. 4,264,424 issued Apr. 28, 1981 to Niedrach. This patent describes a pH electrode comprising a ceramic membrane defining an internal chamber within which is confined a buffered halide solution. A silver/halide electrode is immersed in the buffered halide solution and electrically connected to a similar silver/silver halide electrode immersed in a halide solution which is in contact, via a porous ceramic plug, with the solution whose pH is to be measured.
Niedrach's electrode suffers from a number of disadvantages. Firstly, the silver/silver halide electrodes are exposed to the high temperature of the solution whose pH is being measured and are thus subject to thermal hydrolysis. Also, if Niedrach's pH electrode is used for measuring a solution under a high and variable pressure, any pressure changes may lead to flow of solution through the porous zirconia junction and consequent mixing of the buffered solution in the reference electrode with the solution being measured. Such physical movement of the solution may produce errors in the potentials recorded by the electrode.
There is thus a need for a pH electode which can measure pH's at elevated temperatures and pressures without exposing the measuring electrodes to high temperatures and which does not suffer from spurious readings caused by streaming of fluid if pressure changes occur in the liquid whose pH is being measured. The instant invention seeks to provide such a pH electrode.